Oxidation Catalyst for a Compression Ignition Engine

ABSTRACT

An oxidation catalyst for treating an exhaust gas from a compression ignition engine, which oxidation catalyst comprises: a substrate; a first washcoat region comprising palladium (Pd) and a first support material comprising cerium oxide; and a second washcoat region comprising platinum (Pt) and a second support material.

FIELD OF THE INVENTION

The invention relates to an oxidation catalyst for a compressionignition engine, particularly a diesel engine, and its uses. Theinvention also relates to methods involving the oxidation catalyst. Theinvention further relates to an exhaust system comprising the oxidationcatalyst, and to a vehicle comprising the oxidation catalyst or theexhaust system.

BACKGROUND TO THE INVENTION

Compression ignition engines, such as diesel engines, produce an exhaustemission that generally contains at least four classes of pollutant thatare legislated against by inter-governmental organisations throughoutthe world: carbon monoxide (CO), unburned hydrocarbons (HCs), oxides ofnitrogen (NO_(x)) and particulate matter (PM). As emissions standardsfor permissible emission of pollutants from compression ignitionengines, particularly vehicular engines, become progressively tightened,there is a need to provide improved catalysts and exhaust systems thatare able to meet these standards and which are cost-effective.

Oxidation catalysts comprising platinum group metals have been used totreat carbon monoxide (CO) and hydrocarbons (HCs), including thevolatile organic fraction (VOF) of particulate matter (PM), in exhaustemissions produced by compression ignition engines. Such catalysts treatcarbon monoxide (CO) by oxidising it to carbon dioxide (CO₂), and treathydrocarbons (HCs) by oxidising them to water (H₂O) and carbon dioxide(CO₂). Some platinum group metals, particularly when supported on arefractory oxide, can also promote the oxidation of nitric oxide (NO) tonitrogen dioxide (NO₂).

It has been found that platinum (Pt) and palladium (Pd) are each able tooxidise carbon monoxide (CO) and hydrocarbons (HCs) in an exhaust gasfrom a compression ignition engine. Palladium is generally cheaper thanplatinum, but is less active toward CO and HCs (e.g. Pd has a higherlight-off temperature for CO and HCs than Pt). Palladium is also moresusceptible to poisoning by sulfur in fuel compared to platinum.However, platinum-based oxidation catalysts have been found to generatenitrous oxide (N₂O) by reduction of NO_(x) (Catalysis Today 26 (1995)185-206).

Current legislation for regulating engine emissions does not limitnitrous oxide (N₂O) because it is regulated separately as a greenhousegas (GHG). Nevertheless, it is desirable for emissions to containminimal nitrous oxide (N₂O). The US Environmental Protection Agency hasstated that the impact of 1 pound of nitrous oxide (N₂O) in warming theatmosphere is over 300 times that of 1 pound of carbon dioxide (CO₂).Nitrous oxide (N₂O) is also an ozone-depleting substance (ODS). It hasbeen estimated that nitrous oxide (N₂O) molecules stay in the atmospherefor about 120 years before being removed or destroyed.

Typically, an exhaust gas of a compression ignition engine will betreated using an exhaust system where an oxidation catalyst has beencombined with at least one other emissions control device. In general,the emissions control device will not remove any nitrous oxide (N₂O)generated by the oxidation catalyst, even when the emissions controldevice receives treated exhaust gas from an outlet of the oxidationcatalyst (i.e. the emissions control device is downstream of theoxidation catalyst).

SUMMARY OF THE INVENTION

An object of the invention is to provide an oxidation catalyst fortreating an exhaust gas produced by a compression ignition engine, suchas a diesel engine, which catalyst has advantageous oxidative activitytoward carbon monoxide (CO) and/or hydrocarbons (HCs). In particular,the oxidation catalyst of the invention has excellent CO oxidationactivity at low temperatures (i.e. a low light off temperature for CO(e.g. a low T₅₀)).

The oxidation catalyst of the invention may additionally oralternatively provide the following advantages: (i) it does not, in use,generate or produce a substantial amount of nitrous oxide (N₂O); (ii) itcan act as a passive NO_(x) adsorber (PNA); and/or (iii) it can modulatethe NO_(x) content of an exhaust gas for a downstream emissions controldevice.

The invention provides an oxidation catalyst for treating an exhaust gasfrom a compression ignition engine, which oxidation catalyst comprises:a substrate; a first washcoat region comprising palladium (Pd) and afirst support material comprising cerium oxide; and a second washcoatregion comprising platinum (Pt) and a second support material.

The invention also provides an exhaust system for a compression ignitionengine, which exhaust system comprises an oxidation catalyst and anemissions control device, wherein the oxidation catalyst comprises: asubstrate; a first washcoat region comprising palladium (Pd) and a firstsupport material comprising cerium oxide; and a second washcoat regioncomprising platinum (Pt) and a second support material.

Also provided by the invention is a vehicle comprising a compressionignition engine and either an oxidation catalyst of the invention or anexhaust system of the invention.

The invention also provides several uses of the oxidation catalyst andmethods involving the oxidation catalyst.

A first method aspect of the invention relates to a method of treatingan exhaust gas from a compression ignition engine, which methodcomprises contacting the exhaust gas with an oxidation catalystcomprising: a substrate; a first washcoat region comprising palladium(Pd) and a first support material comprising cerium oxide; and a secondwashcoat region comprising platinum (Pt) and a second support material.Generally, the method of treating an exhaust gas from a compressionignition engine is a method of treating (e.g. oxidising) carbon monoxide(CO) and hydrocarbons (HCs) in an exhaust gas from a compressionignition engine, preferably without producing a substantial amount ofnitrous oxide (N₂O).

A second method aspect of the invention relates to a method ofmodulating the content of NO_(x) in an exhaust gas from a compressionignition engine for an emissions control device, which method comprises:(a) controlling the NO_(x) content of an exhaust gas by contacting theexhaust gas with an oxidation catalyst to produce a treated exhaust gas;and (b) passing the treated exhaust gas to an emissions control device;wherein the oxidation catalyst comprises: a substrate; a first washcoatregion comprising palladium (Pd) and a first support material comprisingcerium oxide; and a second washcoat region comprising platinum (Pt) anda second support material. Typically, the method is also a method oftreating an exhaust gas from a compression ignition engine andmodulating the content of NO_(x) in the exhaust gas for an emissionscontrol device. The method may further relate to a method of treating(e.g. oxidising) carbon monoxide (CO) and hydrocarbons (HCs) in anexhaust gas from a compression ignition engine and modulating thecontent of NO_(x) in the exhaust gas for an emissions control device,preferably without producing a substantial amount of nitrous oxide(N₂O).

A first use aspect of the invention relates to the use of an oxidationcatalyst to treat an exhaust gas from a compression ignition engine,optionally in combination with an emissions control device, wherein theoxidation catalyst comprises: a substrate; a first washcoat regioncomprising palladium (Pd) and a first support material comprising ceriumoxide; and a second washcoat region comprising platinum (Pt) and asecond support material. Generally, the oxidation catalyst is used totreat (e.g. oxidise) carbon monoxide (CO) and hydrocarbons (HCs) in anexhaust gas from a compression ignition engine, preferably withoutproducing a substantial amount of nitrous oxide (N₂O), optionally incombination with an emissions control device. Thus, the oxidationcatalyst can be used to treat (e.g. oxidise) carbon monoxide (CO) andhydrocarbons (HCs) in an exhaust gas from a compression ignition engineand produce a treated exhaust gas comprising substantially no nitrousoxide (N₂O). Typically, the treated exhaust gas is then passed onto anemissions control device.

A second use aspect of the invention relates to the use of an oxidationcatalyst as a passive NO_(x) absorber (PNA) in an exhaust gas from acompression ignition engine optionally in combination with an emissionscontrol device, wherein the oxidation catalyst comprises: a substrate; afirst washcoat region comprising palladium (Pd) and a first supportmaterial comprising cerium oxide; and a second washcoat regioncomprising platinum (Pt) and a second support material. Generally, theoxidation catalyst is used as a passive NO_(x) absorber (PNA) and totreat (e.g. oxidise) carbon monoxide (CO) and hydrocarbons (HCs) in anexhaust gas from a compression ignition engine, preferably withoutproducing a substantial amount of nitrous oxide (N₂O), optionally incombination with an emissions control device.

In a third use aspect, the invention relates to the use of an oxidationcatalyst to modulate the content of NO_(x) in an exhaust gas from acompression ignition engine for an emissions control device, wherein theoxidation catalyst comprises a substrate, the first washcoat region anda second washcoat region, wherein the first washcoat region comprisespalladium (Pd) and a first support material comprising cerium oxide, andwherein the second washcoat region comprises platinum (Pt) and a secondsupport material.

A fourth use aspect relates to the use of an oxidation catalyst in theregeneration of an emissions control device having a filteringsubstrate, wherein the oxidation catalyst comprises a substratemonolith, the first washcoat region and a second washcoat region,wherein the first washcoat region comprises palladium (Pd) and a firstsupport material comprising cerium oxide, and wherein the secondwashcoat region comprises platinum (Pt) and a second support material.

In a fifth use aspect, the invention relates to the use of a firstwashcoat region in an oxidation catalyst (e.g. in an oxidation catalystcomprising the first washcoat region) to reduce or prevent production ofN₂O in an exhaust gas from a compression ignition engine, wherein theoxidation catalyst comprises a substrate, the first washcoat region anda second washcoat region, wherein the first washcoat region comprisespalladium (Pd) and a first support material comprising cerium oxide, andwherein the second washcoat region comprises platinum (Pt) and a secondsupport material. Typically, the oxidation catalyst is used to reduce orprevent production of N₂O from NO_(x) (e.g. by the second washcoatregion that comprises platinum) in an exhaust gas from a compressionignition engine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of the amount of N₂O (ppm) in an exhaust gas thathas been passed over a catalyst (Example 1=dashed line; Example 2=fullline) and which was produced by an engine run over an MVEG cycle.

FIG. 2 is a graph showing the effect of palladium loading on the amountof NO_(x) stored in g L⁻¹ (y-axis) at various temperatures in ° C.(x-axis). Palladium was supported in an amount of 1 wt % (▪), 2 wt % (▴)or 4 wt % (●) on ceria, which was loaded in an amount of 2.7 g in⁻³ ontoa substrate.

FIG. 3 is a graph showing the effect of varying both the palladium andcerium loading on the amount of NO_(x) stored in gL⁻¹ (y-axis) atvarious temperatures in ° C. (x-axis). Palladium and cerium were loadedin amounts of 1 wt % Pd and 2.7 g in⁻³ ceria (▴); 2 wt % Pd and 1.35 gin⁻³ ceria (●); 3 wt % Pd and 0.9 g in⁻³ ceria (♦); and 4 wt % Pd and0.675 g in⁻³ ceria (▪).

DETAILED DESCRIPTION OF THE INVENTION

The oxidation catalyst of the invention has been found to possessexcellent CO oxidation activity, especially for compression ignitionengines that produce an exhaust gas containing a relatively highconcentration of CO. The catalyst of the invention contains bothplatinum (Pt) and palladium (Pd) because the combination of theseplatinum group metals provides advantageous oxidative activity. Forexample, platinum can oxidise nitric oxide (NO) to nitrogen dioxide(NO₂) under certain conditions, which may be advantageous for downstreamemissions control devices. It has also been found that the inclusion ofplatinum in a separate washcoat region to a washcoat region comprisingboth palladium and cerium oxide can reduce or prevent the generation ofnitrous oxide (N₂O).

The oxidation catalyst of the invention comprises a first washcoatregion. The first washcoat region comprises, or consists essentially of,palladium (Pd) and a first support material comprising cerium oxide. Thepalladium (Pd) is typically disposed or supported on the first supportmaterial. For example, the palladium (Pd) can be dispersed on the firstsupport material and/or impregnated into the first support material.

The palladium is generally disposed directly onto or is directlysupported by the first support material (e.g. there is no interveningsupport material between the palladium and the first support material).It is preferred that the palladium is in direct contact with the ceriumoxide (i.e. ceria).

Typically, the first support material comprises, or consists essentiallyof, ceria (CeO₂) or ceria-zirconia (CeO₂—ZrO₂), wherein the ceria orceria-zirconia is optionally doped. The ceria-zirconia may be aceria-zirconia solid solution.

The inclusion of a dopant can thermally stabilise the first supportmaterial. It is to be understood that any reference to “doped” in thiscontext refers to a material where the bulk or host lattice of the ceriaor ceria-zirconia is substitution doped or interstitially doped with adopant. In some instances, small amounts of the dopant may be present ata surface of the ceria or ceria-zirconia. However, most of the dopantwill generally be present in the body of the ceria or ceria-zirconia.

When the first support material comprises ceria-zirconia, then typicallythe ceria-zirconia comprises at least 45% by weight ceria, preferably atleast 50% by weight ceria, more preferably at least 55% by weight ceria,such as at least 70% by weight ceria. The ceria-zirconia may furthercomprise a total of 1 to 15% by weight, preferably 2 to 12.5% by weight(e.g. 5 to 10% by weight), of an oxide or oxides of a second rare earthmetal (e.g. the second rare earth metal is not cerium). The second rareearth metal is typically selected from the group consisting of lanthanum(La), praseodymium (Pr) and combinations thereof.

Generally, the ceria-zirconia consists essentially of 20 to 95% byweight of ceria and 5 to 80% by weight of zirconia (e.g. 50 to 95% byweight ceria and 5 to 50% by weight zirconia), preferably 35 to 80% byweight of ceria and 20 to 65% by weight zirconia (e.g. 55 to 80% byweight ceria and 20 to 45% by weight zirconia), even more preferably 45to 75% by weight of ceria and 25 to 55% by weight zirconia.

When the ceria or ceria-zirconia is doped, then the total amount ofdopant is 0.1 to 5% by weight (i.e. % by weight of the ceria or theceria-zirconia). It is preferred that the total amount of dopant is 0.25to 2.5% by weight, more preferably 0.5 to 1.5% by weight (e.g. about 1%by weight). Ceria may be doped with one or more dopant selected from thegroup consisting of zirconium (Zr), titanium (Ti), silicon (Si), yttrium(Y), lanthanum (La), praseodymium (Pr), samarium (Sm), neodymium (Nd)and an oxide thereof. Ceria-zirconia may be doped with one or moredopant selected from the group consisting of titanium (Ti), silicon(Si), yttrium (Y), lanthanum (La), praseodymium (Pr), samarium (Sm),neodymium (Nd) and an oxide thereof.

It is preferred that the ceria or the ceria-zirconia has a high surfacearea. Typically, the ceria or the ceria-zirconia has a surface area of30 to 300 m²/g, preferably 60 to 200 m²/g. The surface area is measuredusing conventional nitrogen physisorption techniques.

It is preferred that the first support material comprises, or consistsessentially of, ceria (CeO₂) or ceria-zirconia (CeO₂—ZrO₂), which is notdoped. The inclusion of a dopant in the first support material maydecrease the catalytic activity of the first washcoat region.

It is preferred that the first support material consists essentially ofceria. More preferably, the first support material consists essentiallyof ceria in a microporous form or a mesoporous form.

Typically, the first washcoat region comprises an amount of the firstsupport material of 0.1 to 4.5 g in⁻³ (e.g. 0.25 to 4.2 g in⁻³),preferably 0.3 to 3.8 g in⁻³, still more preferably 0.5 to 3.0 g in⁻³,and even more preferably 0.6 to 2.5 g in⁻³ (e.g. 0.75 to 2.3 g in⁻³).

Generally, the first washcoat region comprises an amount of palladium(Pd) of 0.2 to 15% by weight (e.g. 11.5 to 14% by weight or 12 to 15% byweight), preferably 0.5 to 10% by weight, more preferably 1 to 9% byweight (e.g. 1.5 to 8% by weight), such as 2 to 7% by weight (e.g. 4 to6% by weight). The % by weight in this context is with reference to theamount of the first support material.

Typically, the first washcoat region comprises palladium (Pd) in anamount of 5 to 300 g ft⁻³, more preferably 10 to 250 g ft⁻³, such as 20to 200 g ft⁻³, still more preferably 25 to 175 g ft⁻³, and even morepreferably 35 to 150 g ft⁻³ (e.g. 50 to 125 g ft⁻³).

For example, the first washcoat region may comprise palladium (Pd) in anamount of 50 to 300 g ft⁻³, preferably 100 to 275 g ft⁻³, such as 150 to250 g ft⁻³, more preferably 175 to 200 g ft⁻³. In some instances, arelatively high loading of palladium is beneficial (e.g. for COoxidation activity).

Typically, the first washcoat region comprises a ratio by weight ofpalladium (Pd) to cerium (Ce) of 1:1000 to 1:10, preferably 1:500 to1:15, more preferably 1:250 to 1:25.

It has been found that oxidation catalysts comprising palladium disposedor supported on cerium oxide may possess passive NO_(x) adsorber (PNA)activity. The terms “passive NO_(x) absorber” or “passive NO_(x)adsorber” (PNA) as used herein (and as understood in the art) are usedinterchangeably and refer to the ability of a catalyst to (a) absorb(i.e. adsorb) NO_(x) from an exhaust gas (e.g. from a compressionignition engine) at a first temperature range and (b) release NO_(x) ata second temperature range, wherein the second temperature range ishigher than the first temperature range (e.g. the midpoint of the secondtemperature range is higher than the midpoint of the first temperaturerange). It is preferable that the second temperature range does notoverlap with the first temperature range.

Unlike lean NO_(x) trap catalysts (sometimes referred to as a NO_(x)adsorber catalyst (NAC), a De NO_(x) trap (DNT) catalyst, a NO_(x)storage catalyst, a lean NO_(x) trap (LNT) or a NO_(x) storage/reduction(NSR) catalyst), it is not necessary to alter the proportion of air toreductant (e.g. hydrocarbon, carbon monoxide or hydrogen) in an exhaustgas (e.g. from lean to rich), such as by changing the mode of operationof an engine, to release stored NO_(x) from a PNA. PNAs can be used tostore NO_(x) when exhaust gas temperatures are relatively cool, such asshortly after start-up of a compression ignition engine. NO_(x) storage,and generally also NO_(x) release, occurs at temperatures that are lowerthan the temperature at which significant oxidation of nitric oxide (NO)to nitrogen dioxide (NO₂) by platinum occurs.

Normally, PNA activity would be expected to increase as the number ofactive sites on the catalyst is increased (e.g. by increasing the amountof palladium or the relative amount of palladium to ceria) until othereffects that inhibit contact of NO_(x) with active sites of the catalyststart to compete or dominate. However, it has unexpectedly been foundthat excellent PNA activity can be obtained when the amount of palladiumsupported on cerium oxide is relatively low. In fact, surprisinglylimited additional benefit (in relation to PNA activity) is obtainedwhen the loading of palladium on the support material (e.g. ceria) isgreater than 2% by weight. This finding is advantageous because bothpalladium and cerium oxide are expensive materials. PNA activity can beobtained when the oxidation catalyst has a relatively high loading ofthe first support material (e.g. ceria or ceria-zirconia), particularlyin relation to the loading of palladium.

The first washcoat region preferably comprises an amount of the firstsupport material of 0.5 to 3.5 g in⁻³, more preferably 1 to 3.25 g in⁻³,still more preferably 1.1 to 3.0 g in⁻³ (e.g. 1.25 to 2.75 g in⁻³ or 1.5to 2.75 g in⁻³), and even more preferably 1.25 to 2.5 g in⁻³.

The first washcoat region may comprise an amount of palladium (Pd) of0.25 to 4% by weight (e.g. 0.4 to 3.5% by weight), preferably 0.5 to3.0% by weight (e.g. 0.75 to 2.5% by weight or 1 to 1.75% by weight),and even more preferably 0.75 to 1.5% by weight.

It is advantageous if the first washcoat region comprises an amount ofpalladium less than 2% by weight. It is preferred that the firstwashcoat region comprises an amount of palladium of 0.25 to 1.9% byweight, more preferably 0.4 to 1.8% by weight, such as 0.5 to 1.75% byweight, and even more preferably 0.75 to 1.5% by weight.

It is preferable that the first washcoat region comprises a ratio byweight of palladium (Pd) to cerium (Ce) of 1:1000 to 1:10, preferably1:500 to 1:15, more preferably 1:200 to 1:20.

The first washcoat region may comprise palladium (Pd) in an amount of 5to 120 g ft⁻³, preferably 10 to 100 g ft⁻³, such as 25 to 85 g ft⁻³,still more preferably 35 to 80 g ft⁻³, and even more preferably 50 to 75g ft⁻³.

The first washcoat region may comprise 95% or less of the total weightof palladium in the oxidation catalyst, preferably 80% or less, morepreferably 60% or less.

A low loading of palladium may be advantageous for passive NO_(x)absorber activity or in applications where the oxidation catalyst is acatalytic soot filter (CSF).

More generally, it is possible to modify the activity of the oxidationcatalyst by including other metals, such as catalytically active metals,in the first washcoat region, the second washcoat region or in one ormore additional washcoat region(s) (e.g. a third washcoat region or afourth washcoat region etc.).

For example, the first washcoat region may further comprise a firstcatalytically active metal selected from the group consisting ofplatinum (Pt), gold (Au), ruthenium (Ru), rhodium (Rh), iridium (Ir),silver (Ag) and a combination of two or more thereof.

If a first catalytically active metal is present in the first washcoatregion, then preferably the first catalytically active metal is platinum(Pt) or gold (Au), more preferably the first catalytically active metalis platinum (Pt). When the metal is gold (Au), then the palladium (Pd)and the gold (Au) may be a palladium-gold alloy. Catalysts comprisinggold (Au) can be prepared using the method described in WO 2012/120292by the present Applicant.

When the first washcoat region comprises a first catalytically activemetal, then typically the first washcoat region comprises a total amountof first catalytically active metal of 2 to 150 g ft⁻³, more preferably5 to 125 g ft⁻³, such as 10 to 110 g ft⁻³, still more preferably 25 to100 g ft⁻³, and even more preferably 30 to 75 g ft⁻³ (e.g. 40 to 125 gft⁻³).

When the first washcoat region comprises a first catalytically activemetal, it is preferred that the first washcoat region comprises a totalmolar amount of first catalytically active metal that is less than themolar amount of palladium (Pd). Thus, the first washcoat regioncomprises a ratio of the total molar amount of palladium (Pd) to thetotal molar amount of first catalytically active metal of >1:1 (e.g.Pd:M₁ of 20:1 to 1.1:1; 10:1 to 1.25:1; 7.5:1 to 2:1; 5:1 to 2.5:1;where M₁ represents the first catalytically active metal).

The first washcoat region may further comprise a hydrocarbon adsorbent.The hydrocarbon adsorbent may be selected from a zeolite, activecharcoal, porous graphite and a combination of two or more thereof. Itis preferred that the hydrocarbon adsorbent is a zeolite.

When the first washcoat region comprises a hydrocarbon adsorbent, thentypically the total amount of hydrocarbon adsorbent is 0.05 to 3.00 gin⁻³, particularly 0.10 to 2.00 g in⁻³, more particularly 0.2 to 0.8 gin⁻³.

When the hydrocarbon adsorbent is a zeolite, then preferably the zeoliteis a medium pore zeolite (e.g. a zeolite having a maximum ring size ofeight tetrahedral atoms) or a large pore zeolite (e.g. a zeolite havinga maximum ring size of ten tetrahedral atoms).

Examples of suitable zeolites or types of zeolite include faujasite,clinoptilolite, mordenite, silicalite, ferrierite, zeolite X, zeolite Y,ultrastable zeolite Y, AEI zeolite, ZSM-5 zeolite, ZSM-12 zeolite,ZSM-20 zeolite, ZSM-34 zeolite, CHA zeolite, SSZ-3 zeolite, SAPO-5zeolite, offretite, a beta zeolite or a copper CHA zeolite. The zeoliteis preferably ZSM-5, a beta zeolite or a Y zeolite.

In general, it is preferred that the first washcoat region does notcomprise a zeolite, more preferably the first washcoat region does notcomprise a hydrocarbon adsorbent. Thus, the first washcoat region may besubstantially free of hydrocarbon adsorbent or zeolite. If the oxidationcatalyst comprises a hydrocarbon adsorbent, such as a zeolite, thenpreferably the second washcoat region and/or a third washcoat regioncomprises the hydrocarbon adsorbent or zeolite.

Typically, the first washcoat region consists essentially of palladium(Pd), the first catalytically active metal, the first support materialand optionally a hydrocarbon adsorbent. More preferably, the firstwashcoat region consists essentially of palladium (Pd), the firstcatalytically active metal and the first support material.

Generally, the first washcoat region does not comprise a firstcatalytically active metal as described herein (i.e. palladium is theonly active metal for catalysis in the first washcoat region). The firstsupport material may, however, include one or more other materials, butin general such materials are included to stabilise the first supportmaterial (e.g. they form part of the bulk composition of the supportmaterial) and they are not by themselves catalytically active.Preferably, the first washcoat region is substantially free of or doesnot comprise platinum. More preferably, the first washcoat regionconsists essentially of palladium (Pd) and the first support material.

The function of a passive NO_(x) adsorber (PNA) is different to a leanNO_(x) trap catalyst. It is therefore unnecessary to include materialsin the oxidation catalyst that function as a NO_(x) adsorber. SuchNO_(x) adsorber materials store NO_(x) when the exhaust gas is lean andrelease NO_(x) when the exhaust gas is rich.

In general, it is preferred that the first washcoat region issubstantially free of or does not comprise a NO_(x) adsorber materialfor a lean NO_(x) trap catalyst. NO_(x) adsorber materials for a leanNO_(x) trap catalyst typically comprise an alkali metal (e.g. Li, Na,K), an alkaline earth metal (e.g. Mg, Ca, Sr, Ba) and/or a rare earthmetal.

The first washcoat region typically is substantially free of or does notcomprise an alkali metal (e.g. Li, Na, K), an alkaline earth metal (e.g.Mg, Ca, Sr, Ba) and/or a rare earth metal, particularly a rare earthmetal selected from the group consisting of lanthanum (La), yttrium (Y)and a combination thereof. The general exclusion relating to a rareearth metal does not apply to the ceria or ceria-zirconia that is partof the first support material.

Generally, the first washcoat region is substantially free of or doesnot comprise rhodium (Rh).

It is preferred in general that the first washcoat region does notcomprise rhodium (Rh), a hydrocarbon adsorbent, an alkali metal (e.g.Li, Na, K), an alkaline earth metal (e.g. Mg, Ca, Sr, Ba) and a rareearth metal, particularly a rare earth metal selected from the groupconsisting of lanthanum (La), yttrium (Y) and a combination thereof.

The second washcoat region of the invention comprises platinum (Pt) anda second support material. The second washcoat region and first washcoatregion generally have different compositions. It has been found that thegeneration of N₂O by a washcoat region containing Pt (e.g. the secondwashcoat region) can be reduced or prevented when it is combined withthe first washcoat region.

The second washcoat region typically comprises platinum disposed orsupported on the second support material. Platinum may be dispersed onthe second support material and/or impregnated into the second supportmaterial.

Typically, the second washcoat region comprises an amount of platinum(Pt) of 0.2 to 15% by weight, preferably 0.5 to 10% by weight, morepreferably 1 to 9% by weight (e.g. 1.5 to 8% by weight), such as 2 to 7%by weight (e.g. 4 to 6% by weight). The % by weight in this context iswith reference to the amount of the second support material.

The second washcoat region typically comprises platinum (Pt) in anamount of 5 to 300 g ft⁻³, more preferably 10 to 250 g ft⁻³, such as 20to 200 g ft⁻³, still more preferably 25 to 175 g ft⁻³, and even morepreferably 35 to 150 g ft⁻³ (e.g. 50 to 125 g ft⁻³).

The second washcoat region may comprise 50% or more of the total weightof platinum in the oxidation catalyst, preferably 70% or more, morepreferably 90% or more.

In the second washcoat region, platinum may be the only catalyticallyactive metal. Thus, for example, the second washcoat region does notinclude a second catalytically active metal as defined below.

However, the activity of the oxidation catalyst may be modified byincluding other metals, such as a second catalytically active metal, inthe second washcoat region. Thus, the second washcoat region may furthercomprise a second catalytically active metal selected from the groupconsisting of palladium (Pd), gold (Au), ruthenium (Ru), rhodium (Rh),iridium (Ir), silver (Ag) and a combination of two or more thereof. Itis preferred that the second catalytically active metal is palladium(Pd).

The second catalytically active metal may be disposed or supported onthe second support material. Thus, the second catalytically active metalcan be dispersed on the second support material and/or impregnated intothe second support material.

When the second washcoat region comprises a second catalytically activemetal, then typically the second washcoat region comprises a totalamount of second catalytically active metal of 2 to 150 g ft⁻³, morepreferably 5 to 125 g ft⁻³, such as 10 to 110 g ft⁻³, still morepreferably 20 to 100 g ft⁻³, and even more preferably 30 to 75 g ft⁻³(e.g. 40 to 125 g ft⁻³).

When the second washcoat region comprises a second catalytically activemetal, then it is preferred that the second washcoat region comprises atotal molar amount of second catalytically active metal(s) that is lessthan the molar amount of platinum (Pt). Thus, second washcoat regioncomprises a ratio of the molar amount of platinum (Pt) to the totalmolar amount of second catalytically active metal of >1:1 (e.g. Pt:M₂ of20:1 to 1.1:1; 10:1 to 1.25:1; 7.5:1 to 2:1; 5:1 to 2.5:1; where M₂represents the catalytically active metal).

Generally, it is preferred that the second washcoat region comprises, orconsists essentially of, platinum (Pt), palladium (Pd) and a secondsupport material. Typically, the second washcoat region has a ratio bymass of platinum (Pt) to palladium (Pd) of 10:1 to 1:3, more preferably8.5:1 to 1:2.5, such as 7.5:1 to 1:2 (e.g. 7:1 to 1:1.5), still morepreferably 6:1 to 1:1.25 (e.g. 5:1 to 1:1).

In the second washcoat region, the mass of platinum (Pt) is typicallygreater than the mass of palladium (Pd). Advantageous oxidative activitymay be obtained when there is more platinum than palladium in the secondwashcoat region. Thus, the second washcoat region preferably has a ratioby mass of platinum (Pt) to palladium (Pd) of 10:1 to 1.25:1, morepreferably 8:1 to 1.5:1, such as 7:1 to 1.75:1, and still morepreferably 6:1 to 2:1.

Typically, the second support material comprises, or consistsessentially of, a refractory metal oxide. Refractory metal oxidessuitable for use as a catalytic component of an oxidation catalyst for acompression ignition engine are well known in the art.

The refractory metal oxide is preferably selected from the groupconsisting of alumina, silica, titania, zirconia, ceria and mixed orcomposite oxides of two or more thereof. More preferably, the refractorymetal oxide is selected from alumina, silica and mixed or compositeoxides thereof. Even more preferably, the refractory metal oxide isselected from alumina, silica-alumina and a mixture of alumina andceria.

When the refractory metal oxide is a mixed or composite oxide ofalumina, such as silica-alumina or a mixture of alumina and ceria, thenpreferably the mixed or composite oxide of alumina comprises at least 50to 99% by weight of alumina, more preferably 70 to 95% by weight ofalumina, even more preferably 75 to 90% by weight of alumina.

For the avoidance of doubt, the alumina or a mixed or composite oxidecomprising alumina is not a modified alumina incorporating a heteroatomcomponent, particularly a modified alumina incorporating a heteroatomcomponent that comprises, or consists essentially of, an alumina dopedwith a heteroatom component or an alkaline earth metal aluminate. Inthis context, the heteroatom component comprises silicon, magnesium,barium, lanthanum, cerium, titanium, or zirconium or a combination oftwo or more thereof.

It is preferred that the refractory metal oxide is alumina. The aluminacan be α-Al₂O₃, β-Al₂O₃, or γ-Al₂O₃. Preferably the alumina comprises,or consists essentially of, γ-Al₂O₃.

More preferably, the second washcoat region comprises, or consistsessentially of, platinum, palladium and a second support material,wherein the second support material comprises, or consists essentiallyof, alumina.

Typically, the second washcoat region comprises an amount of the secondsupport material of 0.1 to 3.5 g in⁻³, preferably 0.2 to 2.5 g in⁻³,still more preferably 0.3 to 2.0 g in⁻³, and even more preferably 0.5 to1.75 g in⁻³ (e.g. 0.75 to 1.5 g in⁻³).

The second washcoat region may further comprise a hydrocarbon adsorbent.The hydrocarbon adsorbent may be selected from a zeolite, activecharcoal, porous graphite and a combination of two or more thereof. Itis preferred that the hydrocarbon adsorbent is a zeolite, morepreferably a zeolite as defined above.

The second washcoat region typically comprises an amount of hydrocarbonadsorbent of 0.05 to 3.00 g in⁻³, particularly 0.10 to 2.00 g in⁻³, moreparticularly 0.2 to 0.8 g in⁻³.

The second washcoat region may further comprise an oxygen storagematerial. Such materials are well-known in the art. The second washcoatregion may comprise an oxygen storage material in a total amount of 0.1to 10% (e.g. 0.25 to 2.5%, or 0.5 to 1%) of the total amount of thesecond support material.

The oxygen storage material may be selected from ceria (CeO₂) andceria-zirconia (CeO₂—ZrO₂), such as a ceria-zirconia solid solution.When the oxygen storage material is selected from ceria andceria-zirconia, then preferably the oxygen storage material is either(a) ceria when the first support material comprises, or consistsessentially of, ceria-zirconia, or (b) ceria-zirconia when the firstsupport material comprises, or consists essentially of, ceria.

Generally, it is preferable that the second washcoat region consistsessentially of platinum, palladium, the second support material andoptionally a zeolite.

It is preferred that the second washcoat region, or the oxidationcatalyst itself, is substantially free of or does not comprise a NO_(x)adsorber material for a lean NO_(x) trap catalyst. NO_(x) adsorbermaterials for a lean NO_(x) trap catalyst typically comprise an alkalimetal (e.g. Li, Na, K), an alkaline earth metal (e.g. Mg, Ca, Sr, Ba)and/or a rare earth metal. Cerium and cerium oxide are not considered tobe a NO_(x) adsorber material in this context.

The second washcoat region typically is substantially free of or doesnot comprise an alkali metal (e.g. Li, Na, K), an alkaline earth metal(e.g. Mg, Ca, Sr, Ba) and/or a rare earth metal, particularly a rareearth metal selected from the group consisting of lanthanum (La),yttrium (Y) and a combination thereof.

Generally, the second washcoat region or the oxidation catalyst of theinvention is substantially free of or does not comprise rhodium (Rh).

It is preferred in general that the second washcoat region does notcomprise rhodium (Rh), a hydrocarbon adsorbent, an alkali metal (e.g.Li, Na, K), an alkaline earth metal (e.g. Mg, Ca, Sr, Ba) and a rareearth metal, particularly a rare earth metal selected from the groupconsisting of lanthanum (La), yttrium (Y) and a combination thereof.

In general, the first support material and/or the second supportmaterial is in particulate form. Each support material may have a d₉₀particle size of ≤20 μm (as determined by conventional laser diffractiontechniques). The particle size distribution of the support material isselected to aid adhesion to the substrate. The particles are generallyobtained by milling.

Typically, the oxidation catalyst comprises a total amount (by mass) ofplatinum and palladium of 1.0 to 10.0 g. The total amount of platinumand palladium that is used depends on, amongst other things, the size ofthe substrate and the intended application of the oxidation catalyst.

Generally, the total amount of platinum in the first washcoat region andthe second washcoat region to the total amount of palladium in the firstwashcoat region and the second washcoat region have a ratio (by mass) of20:1 to 1:20. Thus, the ratio by mass of platinum to palladium containedin both the first washcoat region and the second washcoat region can be20:1 to 1:20. Preferably, the ratio is 10:1 to 1:10 (e.g. 8:1 to 1:8),more preferably the ratio is 7.5:1 to 1:7.5, such as 5:1 to 1:5, stillmore preferably the ratio is 4:1 to 1:4 (e.g. 3:1 to 1:3), such as 2.5:1to 1:2.5 (e.g. 2:1 to 1:2).

Typically, the oxidation catalyst comprises a total amount of platinum(Pt) and a total amount of palladium (Pd) in a ratio (by mass) of≥1:3.5. It is preferred that the ratio (by mass) is ≥1:2.5, morepreferably 1:2, particularly ≥1:1.5, such as ≥1:1.

Oxidation catalysts of the invention where the total amount (by mass) ofpalladium (Pd) is less than the total amount (by mass) of platinum (Pt),typically where each total amount refers to the combined amount ofpalladium or platinum in the first and second washcoat regions or in theoxidation catalyst as a whole, may have advantageous activity.

The ratio (by mass) of platinum (Pt) to palladium (Pd) is typically 20:1to 1.1:1 (e.g. 15:1 to 1.2:1), preferably the ratio is 10:1 to 1.3:1(e.g. 9:1 to 1.4:1), more preferably 8:1 to 1.5:1, even more preferably7.5:1 to 1.75:1, such as 6:1 to 2:1, and still more preferably 5.5:1 to2.5:1 (e.g. 5:1 to 3:1).

The oxidation catalyst of the invention generally comprises a totalamount of first support material and second support material of 0.2 to 8g in⁻³, preferably 0.4 to 7 g (e.g. 0.5 to 6 g in⁻³), more preferably0.75 to 5 g (e.g. 0.8 to 4 g in⁻³), still more preferably 1.0 to 3 gin⁻³.

The first washcoat region and the second washcoat region are disposed orsupported on the same substrate. Methods of making washcoat regions ofdifferent arrangements are known in the art (see for example WO 99/47260by the present Applicant). However, it is to be understood that certainarrangements of the first washcoat region and the second washcoat regionon the substrate may be particularly advantageous for oxidising CO, forreducing or avoiding the generation of nitrous oxide (N₂O) or forpassive NO_(x) absorber activity.

Generally, the first washcoat region may be disposed directly on to thesubstrate (i.e. the first washcoat region is in contact with a surfaceof the substrate). The second washcoat region may be (a) disposed orsupported on the first washcoat region, (b) disposed directly on to thesubstrate (i.e. the second washcoat region is in contact with a surfaceof the substrate), and/or (c) in contact with the first washcoat region.Alternatively, the second washcoat region may be disposed directly on toan additional washcoat region (e.g. a third washcoat region).

When the second washcoat region is disposed or supported on the firstwashcoat region, the second washcoat region may be disposed directly onto the first washcoat region (i.e. the second washcoat region is incontact with a surface of the first washcoat region) or the secondwashcoat region may be disposed directly on to an additional washcoatregion (e.g. a third washcoat region), where the additional washcoatregion is disposed (e.g. directly or otherwise) or supported on thefirst washcoat region. When the second washcoat region is disposeddirectly on to the substrate, then the second washcoat region may be incontact with the first washcoat region or the first washcoat region andthe second washcoat region may be separated (e.g. by an interveningthird washcoat region or by a gap).

Typically, the second washcoat region is disposed directly on to thesubstrate (i.e. the second washcoat region is in contact with a surfaceof the substrate). The first washcoat region may be (i) disposed orsupported on the second washcoat region, (ii) disposed directly on tothe substrate (i.e. the first washcoat region is in contact with asurface of the substrate), and/or (iii) in contact with the secondwashcoat region. Alternatively, the first washcoat region may bedisposed directly on to an additional washcoat region (e.g. a thirdwashcoat region).

When the first washcoat region is disposed or supported on the secondwashcoat region, the first washcoat region may be disposed directly onto the second washcoat region (i.e. the first washcoat region is incontact with a surface of the second washcoat region) or the firstwashcoat region may be disposed directly on to an additional washcoatregion (e.g. a third washcoat region), where the additional washcoatregion is disposed (e.g. directly or otherwise) or supported on thesecond washcoat region. When the first washcoat region is disposeddirectly on to the substrate, then the first washcoat region may be incontact with the second washcoat region or the second washcoat regionand the first washcoat region may be separated (e.g. by an interveningthird washcoat region or by a gap).

In general, it is possible that both the first washcoat region and thesecond washcoat are not directly disposed on the substrate (i.e. neitherthe first washcoat region nor the second washcoat region is in contactwith a surface of the substrate). Thus, at least one of the firstwashcoat region and the second washcoat region is disposed or supportedon an additional washcoat region (e.g. a third washcoat region). Boththe first washcoat region and the second washcoat region can be disposedor supported on the same additional washcoat region (e.g. a thirdwashcoat region).

Some oxidation catalysts of the invention are described below where thefirst washcoat region and the second washcoat region have “zoned”arrangements. For the avoidance of doubt, these arrangements are generalfeatures of the oxidation catalyst of the invention and may be combinedwith the arrangements of the first and second washcoat regions describedabove.

In a first oxidation catalyst arrangement, the first washcoat region isa first washcoat zone disposed or supported at or near an inlet end ofthe substrate. The second washcoat region may be disposed or supportedupstream or downstream of the first washcoat zone, preferablydownstream. Preferably, the second washcoat region is a second washcoatzone. More preferably, the second washcoat zone is disposed or supporteddownstream of the first washcoat zone.

In a second oxidation catalyst arrangement, the first washcoat region isa first washcoat zone disposed or supported at an outlet end of thesubstrate. The second washcoat region may be disposed or supportedupstream or downstream of the first washcoat zone, preferably upstream.Preferably, the second washcoat region is a second washcoat zone. Morepreferably, the second washcoat zone is disposed or supported upstreamof the first washcoat zone.

In a third oxidation catalyst arrangement, the second washcoat region isa second washcoat zone disposed or supported at an inlet end of thesubstrate. The first washcoat region may be disposed or supportedupstream or downstream of the second washcoat zone, preferablydownstream. Preferably, the first washcoat region is a first washcoatzone. More preferably, the first washcoat zone is disposed or supporteddownstream of the second washcoat zone.

In a fourth oxidation catalyst arrangement, the second washcoat regionis a second washcoat zone disposed or supported at an outlet end of thesubstrate. The first washcoat region may be disposed or supportedupstream or downstream of the second washcoat zone, preferably upstream.Preferably, the first washcoat region is a first washcoat zone. Morepreferably, the first washcoat zone is disposed or supported upstream ofthe second washcoat zone.

The first washcoat zone may adjoin the second washcoat zone. Morepreferably, the first washcoat zone is contact with the second washcoatzone. When the first washcoat zone adjoins the second washcoat zone orthe first washcoat zone is in contact with the second washcoat zone, thefirst washcoat zone and the second washcoat zone may be disposed orsupported on the substrate as a layer, such as a single layer (e.g. asingle layer). Thus, a layer may be formed on the substrate when thefirst and second washcoat zones adjoin or are in contact with oneanother.

The first washcoat zone may be separate from the second washcoat zone.Thus, there may be an intervening additional washcoat zone or region(e.g. a third washcoat zone or region) between the first washcoat zoneand the second washcoat zone, and/or there may be a gap (e.g. a space)between the first washcoat zone and the second washcoat zone.

The first washcoat zone may overlap the second washcoat zone. Thus, anend portion of the first washcoat zone may be disposed or supported onthe second washcoat zone. The first washcoat zone may completely orpartly overlap the second washcoat zone. When the first washcoat zonecompletely overlaps the second washcoat zone, then typically a surfaceof the second washcoat zone (normally a surface in the longitudinalplane of the catalyst, i.e. the plane that is perpendicular to the planeof the inlet and outlet ends of the substrate) is completely covered bythe first washcoat zone.

Alternatively, the second washcoat zone may overlap the first washcoatzone. Thus, an end portion of the second washcoat zone may be disposedor supported on the first washcoat zone. The second washcoat zone maycompletely or partly overlap the first washcoat zone. When the secondwashcoat zone completely overlaps the first washcoat zone, thentypically a surface of the first washcoat zone (normally a surface inthe longitudinal plane of the catalyst, i.e. the plane that isperpendicular to the plane of the inlet and outlet ends of thesubstrate) is completely covered by the second washcoat zone.

Typically, the first washcoat zone has a length of 10 to 90% of thelength of the substrate (e.g. 10 to 45%), preferably 15 to 75% of thelength of the substrate (e.g. 15 to 40%), more preferably 20 to 65%(e.g. 25 to 45%) of the length of the substrate, still more preferably25 to 50%.

The second washcoat zone typically has a length of 10 to 90% of thelength of the substrate (e.g. 10 to 45%), preferably 15 to 75% of thelength of the substrate (e.g. 15 to 40%), more preferably 20 to 65%(e.g. 25 to 45%) of the length of the substrate, still more preferably25 to 50%.

Oxidation catalysts of the invention are described below where the firstwashcoat region and the second washcoat region have “layered”arrangements. For the avoidance of doubt, these arrangements are generalfeatures of the oxidation catalyst of the invention and may be combinedwith any of the arrangements of the first and second washcoat regionsdescribed above.

The first washcoat region may be a first washcoat layer and the secondwashcoat region may be a second washcoat layer. The first washcoat layerand the second washcoat layer may have different lengths, or the firstwashcoat layer and the second washcoat layer may have about the samelength. Generally, the length of the first washcoat layer and the lengthof the second washcoat layer is each substantially uniform.

Typically, at least one of the first washcoat layer and the secondwashcoat layer extends for substantially an entire length of thesubstrate, particularly the entire length of the channels of a substratemonolith. More preferably, the first washcoat layer and the secondwashcoat layer each extend for substantially an entire length of thesubstrate.

In a fifth oxidation catalyst arrangement, the first washcoat layer isdisposed or supported on the second washcoat layer. The first washcoatlayer may be disposed directly on to the second washcoat layer (i.e. thefirst washcoat layer is in contact with a surface of the second washcoatlayer) or the first washcoat layer may be disposed directly on to anadditional washcoat region or layer (e.g. a third washcoat region orlayer), which additional washcoat region or layer is disposed orsupported on the second washcoat layer. Preferably, the entire length ofthe first washcoat layer is disposed or supported on the second washcoatlayer. Thus, the length of the first washcoat layer is less than orequal to the length of the second washcoat layer. More preferably, anend of the first washcoat layer does not extend beyond an end of thesecond washcoat layer (i.e. the ends or boundaries of the first washcoatlayer are within the ends or boundaries of the second washcoat layer).

In the fifth oxidation catalyst arrangement, the first washcoat layermay be disposed or supported on an additional washcoat region (e.g. athird or fourth washcoat region), particularly an additional washcoatlayer or zone. The additional washcoat region may be disposed directlyon to the substrate.

It is preferred in the fifth oxidation catalyst arrangement that thesecond washcoat layer is disposed directly on to the substrate (i.e. thesecond washcoat layer is in contact with a surface of the substrate).

In the fifth oxidation catalyst arrangement, an additional washcoatregion (e.g. a third, fourth or fifth washcoat region), particularly anadditional washcoat layer or zone, may be disposed directly on to thesecond washcoat layer (i.e. the additional washcoat region is in contactwith a surface of the second washcoat layer).

In a sixth oxidation catalyst arrangement, the second washcoat layer isdisposed or supported on the first washcoat layer. The second washcoatlayer may be disposed directly on to the first washcoat layer (i.e. thesecond washcoat layer is in contact with a surface of the first washcoatlayer) or the second washcoat layer may be disposed directly on to anadditional washcoat region or layer (e.g. a third washcoat region orlayer), which additional washcoat region or layer is disposed orsupported on the first washcoat layer. Preferably, the entire length ofthe second washcoat layer is disposed or supported on the first washcoatlayer. Thus, the length of the second washcoat layer is less than orequal to the length of the first washcoat layer. More preferably, an endof the second washcoat layer does not extend beyond an end of the firstwashcoat layer (i.e. the ends or boundaries of the second washcoat layerare within the ends or boundaries of the first washcoat layer).

In the sixth oxidation catalyst arrangement, the second washcoat layermay be disposed or supported on an additional washcoat region (e.g. athird or fourth washcoat region), particularly an additional washcoatlayer or zone. The additional washcoat region may be disposed directlyon to the substrate.

It is preferred in the sixth oxidation catalyst arrangement that thefirst washcoat layer is disposed directly on to the substrate (i.e. thefirst washcoat layer is in contact with a surface of the substrate).

In the sixth oxidation catalyst arrangement, an additional washcoatregion (e.g. a third, fourth or fifth washcoat region), particularly anadditional washcoat layer or zone, may be disposed directly on to thefirst washcoat layer (i.e. the additional washcoat region is in contactwith a surface of the first washcoat layer).

The oxidation catalyst of the invention may comprise a plurality ofwashcoat regions, in addition to the first washcoat region and thesecond washcoat region.

Generally, it is preferred that the oxidation catalyst comprises onlythree or four washcoat regions (including the first washcoat region andthe second washcoat region). Thus, the oxidation catalyst may furthercomprise a third washcoat region (e.g. a third washcoat layer or zone)and a fourth washcoat region (e.g. a fourth washcoat layer or zone. Morepreferably, the oxidation catalyst comprises only three washcoat regions(including the first washcoat region and the second washcoat region).Thus, the oxidation catalyst further comprises a third washcoat region.Even more preferably, the oxidation catalyst comprises only two washcoatregions, which washcoat regions are the first washcoat region and thesecond washcoat region.

Certain oxidation catalyst arrangements are particularly advantageousfor reducing the amount of nitrous oxide (N₂O) that is generated.Oxidation catalyst arrangements where most or all of the exhaust gascomes into contact with the first washcoat region before the secondwashcoat region have been found to generate less N₂O.

It is preferred that the first washcoat region is arranged to contactinlet exhaust gas before the second washcoat region, such as in any oneof the first to fifth oxidation catalyst arrangements, and wherein inthe first to fourth oxidation catalyst arrangements the second washcoatregion is disposed or supported downstream of the first washcoat region.More preferably, the oxidation catalyst has the first oxidation catalystarrangement or the fifth oxidation catalyst arrangement, wherein in thefirst oxidation catalyst arrangement the second washcoat region isdisposed or supported downstream of the first washcoat region.

Advantageous oxidation activity and/or the amount of nitrous oxide (N₂O)produced by the oxidation catalyst can be reduced or prevented when theoxidation catalyst comprises a substrate, and:

-   (a) a first washcoat zone comprising palladium (Pd) and a first    support material comprising cerium oxide; and a second washcoat zone    comprising platinum (Pt) and a second support material; wherein the    second washcoat zone is disposed or supported on the substrate    downstream of the first washcoat zone; or-   (b) a first washcoat layer comprising palladium (Pd) and a first    support material comprising cerium oxide; and a second washcoat    layer comprising platinum (Pt) and a second support material;    wherein the first washcoat layer is disposed or supported on the    second washcoat layer.

Generally, the oxidation catalyst further comprises an additionalwashcoat region (e.g. a third washcoat region), wherein the additionalwashcoat region is arranged to contact inlet exhaust gas before thefirst washcoat region and the second washcoat region, and wherein theadditional washcoat region comprises a hydrocarbon adsorbent. Thus, theadditional washcoat region may be downstream of both the first washcoatregion and the second washcoat region and/or the additional washcoatregion may be disposed or supported on both the first washcoat regionand the second washcoat region (e.g. the additional washcoat region maybe the uppermost layer). The hydrocarbon adsorbent may be present in theadditional washcoat region in an amount as defined above for the firstwashcoat region or the second washcoat region. Preferably, thehydrocarbon adsorbent is a zeolite, such as a zeolite as defined above.

Substrates for supporting oxidation catalysts for treating the exhaustgas of a compression ignition engine are well known in the art. Thesubstrate typically has a plurality of channels (e.g. for the exhaustgas to flow through). Generally, the substrate is a ceramic material ora metallic material.

It is preferred that the substrate is made or composed of cordierite(SiO₂—Al₂O₃—MgO), silicon carbide (SiC), Fe—Cr—Al alloy, Ni—Cr—Al alloy,or a stainless steel alloy.

Typically, the substrate is a monolith (also referred to herein as asubstrate monolith). Such monoliths are well-known in the art. Thesubstrate monolith may be a flow-through monolith or a filteringmonolith.

A flow-through monolith typically comprises a honeycomb monolith (e.g. ametal or ceramic honeycomb monolith) having a plurality of channelsextending therethrough, which channels are open at both ends. When thesubstrate is a flow-through monolith, then the oxidation catalyst of theinvention is typically a diesel oxidation catalyst (DOC) and/or apassive NO_(x) absorber (PNA) or is for use as a diesel oxidationcatalyst (DOC) and/or as a passive NO_(x) absorber (PNA).

A filtering monolith generally comprises a plurality of inlet channelsand a plurality of outlet channels, wherein the inlet channels are openat an upstream end (i.e. exhaust gas inlet side) and are plugged orsealed at a downstream end (i.e. exhaust gas outlet side), the outletchannels are plugged or sealed at an upstream end and are open at adownstream end, and wherein each inlet channel is separated from anoutlet channel by a porous structure. When the substrate is a filteringmonolith, then the oxidation catalyst of the invention is typically acatalysed soot filter (CSF) or is for use as a catalysed soot filter(CSF).

When the monolith is a filtering monolith, it is preferred that thefiltering monolith is a wall-flow filter. In a wall-flow filter, eachinlet channel is alternately separated from an outlet channel by a wallof the porous structure and vice versa. It is preferred that the inletchannels and the outlet channels are arranged in a honeycombarrangement. When there is a honeycomb arrangement, it is preferred thatthe channels vertically and laterally adjacent to an inlet channel areplugged at an upstream end and vice versa (i.e. the channels verticallyand laterally adjacent to an outlet channel are plugged at a downstreamend). When viewed from either end, the alternately plugged and open endsof the channels take on the appearance of a chessboard.

In principle, the substrate may be of any shape or size. However, theshape and size of the substrate is usually selected to optimise exposureof the catalytically active materials in the catalyst to the exhaustgas. The substrate may, for example, have a tubular, fibrous orparticulate form. Examples of suitable supporting substrates include asubstrate of the monolithic honeycomb cordierite type, a substrate ofthe monolithic honeycomb SiC type, a substrate of the layered fibre orknitted fabric type, a substrate of the foam type, a substrate of thecrossflow type, a substrate of the metal wire mesh type, a substrate ofthe metal porous body type and a substrate of the ceramic particle type.

In general, the oxidation catalyst of the invention is for use as (i) apassive NO_(x) absorber (PNA) and/or (ii) a diesel oxidation catalyst(DOC) or a catalysed soot filter (CSF). In practice, catalystformulations employed in DOCs and CSFs are similar. Generally, however,a principle difference between a DOC and a CSF is the substrate ontowhich the catalyst formulation is coated and the total amount ofplatinum, palladium and any other catalytically active metals that arecoated onto the substrate.

The invention also provides an exhaust system comprising the oxidationcatalyst and an emissions control device. In general, the emissionscontrol device is separate to the oxidation catalyst (e.g. the emissionscontrol device has a separate substrate to the substrate of theoxidation catalyst), and preferably the oxidation catalyst is upstreamof the emissions control device.

The exhaust system of the invention may further comprise a fuelsulphur-removal device. A fuel sulphur-removal device can be upstream ordownstream of the oxidation catalyst. Preferably the fuelsulphur-removal device is upstream of the oxidation catalyst. Fuelsulphur-removal devices are known in the art. The oxidation catalyst ofthe invention may be susceptible to deactivation by sulphur.Deactivation of the oxidation catalyst by fuel sulphur can be reduced orprevented when a fuel sulphur-removal device is upstream of theoxidation catalyst. Oxidation catalysts containing platinum oftenoxidise fuel sulphur to SO₂ or may increase sulphate particulateemissions, particularly at higher exhaust temperatures.

The emissions control device may be selected from a diesel particulatefilter (DPF), a NO_(x) adsorber catalyst (NAC), a lean NO_(x) catalyst(LNC), a selective catalytic reduction (SCR) catalyst, a dieseloxidation catalyst (DOC), a catalysed soot filter (CSF), a selectivecatalytic reduction filter (SCRF™) catalyst, and combinations of two ormore thereof. Emissions control devices represented by the terms dieselparticulate filters (DPFs), NO_(x) adsorber catalysts (NACs), leanNO_(x) catalysts (LNCs), selective catalytic reduction (SCR) catalysts,diesel oxidation catalyst (DOCs), catalysed soot filters (CSFs) andselective catalytic reduction filter (SCRF™) catalysts are all wellknown in the art.

Examples of emissions control devices for use with the oxidationcatalyst of the invention or for inclusion in the exhaust system of theinvention are provided below.

A diesel particulate filter is an emissions control device having afiltering substrate. The diesel particulate filter preferably comprisesa substrate, wherein the substrate is a filtering monolith or aflow-through monolith as defined above, preferably a filtering monolith.The substrate may be coated with a catalyst formulation.

The catalyst formulation of the diesel particulate filter may besuitable for oxidising (i) particulate matter (PM) and/or (ii) carbonmonoxide (CO) and hydrocarbons (HCs). When the catalyst formulation issuitable for oxidising PM, then the resulting emissions control deviceis known as a catalysed soot filter (CSF). A catalysed soot filter (CSF)is also an emissions control device having a filtering substrate.Typically, the catalyst formulation of a CSF comprises a noble metal asdefined above and/or platinum and/or palladium.

The catalyst formulation of the diesel particulate filter may be aNO_(x) adsorber composition. When the catalyst formulation is a NO_(x)adsorber composition, the emissions control device is an example of aNO_(x) adsorber catalyst (NAC) (e.g. a NO_(x) adsorber composition on afilter). Emissions control devices where the catalyst formulation is aNO_(x) adsorber composition have been described (see, for example, EP0766993). NO_(x) adsorber compositions are well known in the art (see,for example, EP 0766993 and U.S. Pat. No. 5,473,887). NO_(x) adsorbercompositions are designed to adsorb NO_(x) from lean exhaust gas(lambda >1) and to desorb the NO_(x) when the oxygen concentration inthe exhaust gas is decreased. Desorbed NO_(x) may then be reduced to N₂with a suitable reductant (e.g. engine fuel) and promoted by a catalystcomponent, such as rhodium, of the NO_(x) adsorber composition itself orlocated downstream of the NO_(x) adsorber composition.

In general, NO_(x) adsorber catalysts comprise coated on honeycombflow-through monolith substrates are typically arranged in layeredarrangements. However, multiple layers applied on a filter substrate cancreate backpressure problems. It is highly preferable, therefore, if theNO_(x) absorber catalyst for use in the present invention is a “singlelayer” NO_(x) absorber catalyst. Particularly preferred “single layer”NO_(x) absorber catalysts comprise a first component of rhodiumsupported on a ceria-zirconia mixed oxide or an optionally stabilisedalumina (e.g. stabilised with silica or lanthana or another rare earthelement) in combination with second components which support platinumand/or palladium. The second components comprise platinum and/orpalladium supported on an alumina-based high surface area support and aparticulate “bulk” ceria (CeO₂) component, i.e. not a soluble ceriasupported on a particulate support, but “bulk” ceria capable ofsupporting the Pt and/or Pd as such. The particulate ceria comprises aNO_(x) absorber component and supports an alkaline earth metal and/or analkali metal, preferably barium, in addition to the platinum and/orpalladium. The alumina-based high surface area support can be magnesiumaluminate e.g. MgAl₂O₄, for example.

The preferred “single layer” NAC composition comprises a mixture of therhodium and platinum and/or palladium support components. Thesecomponents can be prepared separately, i.e. pre-formed prior tocombining them in a mixture, or rhodium, platinum and palladium saltsand the supports and other components can be combined and the rhodium,platinum and palladium components hydrolysed preferentially to depositonto the desired support.

Generally, a NO_(x) adsorber composition comprises an alkali metalcomponent, an alkaline earth metal component or a rare earth metalcomponent or a combination of two or more components thereof, whereinthe rare earth metal component comprises lanthanum or yttrium. It ispreferred that the alkali metal component comprises potassium or sodium,more preferably potassium. It is preferred that the alkaline earth metalcomponent comprises barium or strontium, more preferably barium.

The NO_(x) adsorber composition may further comprise a support materialand/or a catalytic metal component. The support material may be selectedfrom alumina, ceria, titania, zirconia and mixtures thereof. Thecatalytic metal component may comprise a metal selected from platinum(Pt), palladium (Pd), rhodium (Rh) and combinations of two or morethereof.

Lean NO_(x) catalysts (LNCs) are well known in the art. Preferred leanNO_(x) catalysts (LNC) comprises either (a) platinum (Pt) supported onalumina or (b) a copper exchanged zeolite, particularly copper exchangedZSM-5.

SCR catalysts are also well known in the art. When the exhaust system ofthe invention comprises an SCR catalyst, then the exhaust system mayfurther comprise an injector for injecting a nitrogenous reductant, suchas ammonia, or an ammonia precursor, such as urea or ammonium formate,preferably urea, into exhaust gas downstream of the catalyst foroxidising carbon monoxide (CO) and hydrocarbons (HCs) and upstream ofthe SCR catalyst. Such injector is fluidly linked to a source of suchnitrogenous reductant precursor, e.g. a tank thereof, andvalve-controlled dosing of the precursor into the exhaust stream isregulated by suitably programmed engine management means and closed loopor open loop feedback provided by sensors monitoring relevant exhaustgas composition. Ammonia can also be generated by heating ammoniumcarbamate (a solid) and the ammonia generated can be injected into theexhaust gas.

Alternatively or in addition to the injector, ammonia can be generatedin situ e.g. during rich regeneration of a NAC disposed upstream of thefilter or by contacting a DOC disposed upstream of the filter withengine-derived rich exhaust gas. Thus, the exhaust system may furthercomprise an engine management means for enriching the exhaust gas withhydrocarbons. The SCR catalyst can then use the hydrocarbons as areductant to reduce NO_(x).

SCR catalysts for use in the present invention promote the reactionsselectively 4NH₃+4NO+O₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO);4NH₃+2NO+2NO₂→4N₂+6H₂O (i.e. 1:1 NH₃:NO_(x); and 8NH₃+6NO₂→7N₂+12H₂O(i.e. 4:3 NH₃:NO_(x)) in preference to undesirable, non-selectiveside-reactions such as 2NH₃+2NO₂→N₂O+3H₂O+N₂.

The SCR catalyst may comprise a metal selected from the group consistingof at least one of Cu, Hf, La, Au, In, V, lanthanides and Group VIIItransition metals, such as Fe, which metal is supported on a refractoryoxide or molecular sieve. Particularly preferred metals are Ce, Fe andCu and combinations of any two or more thereof.

The refractory oxide may be selected from the group consisting of Al₂O₃,TiO₂, CeO₂, SiO₂, ZrO₂ and mixed oxides containing two or more thereof.The non-zeolite catalyst can also include tungsten oxide, e.g.V₂O₅/WO₃/TiO₂, WO_(x)/CeZrO₂, WO_(x)/ZrO₂ or Fe/WO_(x)/ZrO₂.

It is particularly preferred when an SCR catalyst or washcoat thereofcomprises at least one molecular sieve, such as an aluminosilicatezeolite or a SAPO. The at least one molecular sieve can be a small, amedium or a large pore molecular sieve, for example. By “small poremolecular sieve” herein we mean molecular sieves containing a maximumring size of 8, such as CHA; by “medium pore molecular sieve” herein wemean a molecular sieve containing a maximum ring size of 10, such asZSM-5; and by “large pore molecular sieve” herein we mean a molecularsieve having a maximum ring size of 12, such as beta. Small poremolecular sieves are potentially advantageous for use in SCRcatalysts—see for example WO 2008/132452.

Preferred molecular sieves with application as SCR catalysts in thepresent invention are synthetic aluminosilicate zeolite molecular sievesselected from the group consisting of AEI, ZSM-5, ZSM-20, ERI includingZSM-34, mordenite, ferrierite, BEA including Beta, Y, CHA, LEV includingNu-3, MCM-22 and EU-1, preferably AEI or CHA, and having asilica-to-alumina ratio of about 10 to about 50, such as about 15 toabout 40.

At its most basic, an ammonia slip catalyst (ASC) can be an oxidationcatalyst for oxidising ammonia which slips past an upstream SCR or SCRFcatalyst unreacted. The desired reaction (simplified) can be representedby 4NO+4NH₃+O₂→4N₂+6H₂O. Ammonia is a strong smelling compound andpotential irritant to animal mucosal surfaces, e.g. eyes and respiratorypathways, and so its emission to atmosphere should be limited so far aspossible. Possible ammonia slip catalysts include relatively low loadedplatinum group metals, preferably including Pt e.g. 1-15 g/ft³, on asuitable relatively high surface area oxide support, e.g. alumina coatedon a suitable substrate monolith.

In a particularly preferred arrangement, however, the platinum groupmetal and the support material (e.g. comprising a modified aluminaincorporating a heteroatom component) is disposed on a substrate (i.e. asubstrate monolith) in a first layer below an upper, second layeroverlying the first layer. The second layer is a SCR catalyst, selectedfrom any of those mentioned hereinabove, particularly molecular sievescontaining transition metals, such as Cu or Fe. A particularly preferredASC in the layered arrangement comprises CuCHA in the second or upperlayer.

When the substrate of the SCR catalyst is a filtering monolith, then thecatalyst is an SCRF™ catalyst. An SCRF™ catalyst is an emissions controldevice having a filtering substrate.

Generally, SCR catalysts are unable to reduce substantial amounts ofNO_(x) in an exhaust gas shortly after start-up of a compressionignition engine because the exhaust gas temperature (and hence thetemperature of the catalyst) is too low. Lean NO_(x) trap catalystshave, for example, been employed upstream of SCR catalysts, so thatNO_(x) can be stored until the SCR catalyst becomes active at higherexhaust gas temperatures. However, lean NO_(x) trap catalysts are oftenunable to adequately store NO_(x) when there is a large mass flow ofexhaust gas (e.g. when the engine is operated at a high speed cycle).

The NO_(x) content of an exhaust gas directly from a compressionignition engine depends on a number of factors, such as the mode ofoperation of the engine, the temperature of the engine and the speed atwhich the engine is run. However, it is common for an engine to producean exhaust gas where NO_(x) content is 85 to 95% (by volume) nitricoxide (NO) and 5 to 15% (by volume) nitrogen dioxide (NO₂). The NO:NO₂ratio is typically from 19:1 to 17:3. However, it is generallyfavourable for the NO₂ content to be much higher for selective catalyticreduction (SCR) catalysts to reduce NO_(x) or to regenerate an emissionscontrol device having a filtering substrate by burning off particulatematter. The PNA activity of the oxidation catalyst can be used tomodulate the NO_(x) content of an exhaust gas from a compressionignition engine.

The PNA activity of the oxidation catalyst of the present inventionallows NO_(R), particularly NO_(x) to be stored at low exhausttemperatures. At higher exhaust gas temperatures, the oxidation catalystis able to oxidise NO to NO₂. It is therefore advantageous to combinethe oxidation catalyst of the invention with certain types of emissionscontrol devices as part of an exhaust system.

In a first exhaust system embodiment, the exhaust system comprises theoxidation catalyst of the invention, preferably as a PNA and/or a DOC,and a selective catalytic reduction (SCR) catalyst. This embodiment alsorelates to the use of the oxidation catalyst for treating an exhaust gasfrom a compression ignition engine in combination with a selectivecatalytic reduction filter (SCRF™) catalyst, preferably wherein theoxidation catalyst is, or is for use as, a diesel oxidation catalyst.The oxidation catalyst of the invention is typically followed by (e.g.is upstream of) the selective catalytic reduction (SCR) catalyst. Anitrogenous reductant injector may be arranged between the oxidationcatalyst and the selective catalytic reduction (SCR) catalyst. Thus, theoxidation catalyst may be followed by (e.g. is upstream of) anitrogenous reductant injector, and the nitrogenous reductant injectormay be followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst.

A second exhaust system embodiment comprises the oxidation catalyst ofthe invention, preferably as a PNA and/or a DOC, and a selectivecatalytic reduction filter (SCRF™) catalyst. This embodiment alsorelates to the use of the oxidation catalyst for treating an exhaust gasfrom a compression ignition engine in combination with a selectivecatalytic reduction filter (SCRF™) catalyst, preferably wherein theoxidation catalyst is, or is for use as, a diesel oxidation catalyst.The oxidation catalyst of the invention is typically followed by (e.g.is upstream of) the selective catalytic reduction filter (SCRF™)catalyst. A nitrogenous reductant injector may be arranged between theoxidation catalyst and the selective catalytic reduction filter (SCRF™)catalyst. Thus, the oxidation catalyst may be followed by (e.g. isupstream of) a nitrogenous reductant injector, and the nitrogenousreductant injector may be followed by (e.g. is upstream of) theselective catalytic reduction filter (SCRF™) catalyst.

In a third exhaust system embodiment, the exhaust system comprises theoxidation catalyst of the invention, preferably as a PNA and/or a DOC,and either a diesel particulate filter (DPF) or a catalysed soot filter(CSF). This embodiment also relates to the use of the oxidation catalystfor treating an exhaust gas from a compression ignition engine incombination with a diesel particulate filter or a catalysed soot filter,preferably wherein the oxidation catalyst is, or is for use as, a dieseloxidation catalyst. The oxidation catalyst is typically followed by(e.g. is upstream of) the diesel particulate filter or the catalysedsoot filter (CSF). Thus, for example, an outlet of the oxidationcatalyst is connected to an inlet of the diesel particulate filter orthe catalysed soot filter.

In a fourth exhaust system embodiment, the exhaust system comprises adiesel oxidation catalyst and the oxidation catalyst of the invention,preferably as a catalysed soot filter (CSF). The embodiment furtherrelates to the use of the oxidation catalyst for treating an exhaust gasfrom a compression ignition engine in combination with a dieseloxidation catalyst (DOC), preferably wherein the oxidation catalyst is,or is for use as, a catalysed soot filter. Typically, the dieseloxidation catalyst (DOC) is followed by (e.g. is upstream of) theoxidation catalyst of the invention. Thus, an outlet of the dieseloxidation catalyst is connected to an inlet of the oxidation catalyst ofthe invention.

A fifth exhaust system embodiment relates to an exhaust systemcomprising the oxidation catalyst of the invention, preferably as a PNAand/or a DOC, a diesel particulate filter or a catalysed soot filter(CSF), and a selective catalytic reduction (SCR) catalyst. TheDOC/DPF/SCR or DOC/CSF/SCR arrangement is a preferred exhaust system fora light-duty diesel vehicle. This embodiment also relates to the use ofthe oxidation catalyst for treating an exhaust gas from a compressionignition engine in combination with either a diesel particulate filteror a catalysed soot filter (CSF), and a selective catalytic reduction(SCR) catalyst, preferably wherein the oxidation catalyst is, or is foruse as, a diesel oxidation catalyst. The oxidation catalyst is typicallyfollowed by (e.g. is upstream of) the diesel particulate filter or thecatalysed soot filter (CSF). The DPF or CSF is typically followed by(e.g. is upstream of) the selective catalytic reduction (SCR) catalyst.A nitrogenous reductant injector may be arranged between the DPF or CSFand the selective catalytic reduction (SCR) catalyst. Thus, the DPF orCSF may be followed by (e.g. is upstream of) a nitrogenous reductantinjector, and the nitrogenous reductant injector may be followed by(e.g. is upstream of) the selective catalytic reduction (SCR) catalyst.

A sixth exhaust system embodiment relates to an exhaust systemcomprising a diesel oxidation catalyst (DOC), the oxidation catalyst ofthe invention, preferably as a catalysed soot filter (CSF), and aselective catalytic reduction (SCR) catalyst. This is also a DOC/CSF/SCRarrangement. A further aspect of this embodiment relates to the use ofthe oxidation catalyst for treating an exhaust gas from a compressionignition engine in combination with a diesel oxidation catalyst (DOC)and a selective catalytic reduction (SCR) catalyst, preferably whereinthe oxidation catalyst is, or is for use as, a catalysed soot filter(CSF). The diesel oxidation catalyst (DOC) is typically followed by(e.g. is upstream of) the oxidation catalyst of the invention. Theoxidation catalyst of the invention is typically followed by (e.g. isupstream of) the selective catalytic reduction (SCR) catalyst. Anitrogenous reductant injector may be arranged between the oxidationcatalyst and the selective catalytic reduction (SCR) catalyst. Thus, theoxidation catalyst may be followed by (e.g. is upstream of) anitrogenous reductant injector, and the nitrogenous reductant injectormay be followed by (e.g. is upstream of) the selective catalyticreduction (SCR) catalyst.

In a seventh exhaust system embodiment, the exhaust system comprises theoxidation catalyst of the invention, preferably as a PNA and/or a DOC, aselective catalytic reduction (SCR) catalyst and either a catalysed sootfilter (CSF) or a diesel particulate filter (DPF). This embodiment alsorelates to the use of the oxidation catalyst for treating an exhaust gasfrom a compression ignition engine in combination with a selectivecatalytic reduction (SCR) catalyst and either a catalysed soot filter(CSF) or a diesel particulate filter (DPF), preferably wherein theoxidation catalyst is, or is for use as, a diesel oxidation catalyst.

In the seventh exhaust system embodiment, the oxidation catalyst of theinvention is typically followed by (e.g. is upstream of) the selectivecatalytic reduction (SCR) catalyst. A nitrogenous reductant injector maybe arranged between the oxidation catalyst and the selective catalyticreduction (SCR) catalyst. Thus, the oxidation catalyst may be followedby (e.g. is upstream of) a nitrogenous reductant injector, and thenitrogenous reductant injector may be followed by (e.g. is upstream of)the selective catalytic reduction (SCR) catalyst. The selectivecatalytic reduction (SCR) catalyst are followed by (e.g. are upstreamof) the catalysed soot filter (CSF) or the diesel particulate filter(DPF).

In any of the first, second or fifth to seventh exhaust systemembodiments described hereinabove containing a SCR catalyst (includingSCRF™ catalyst), an ASC catalyst can be disposed downstream from the SCRcatalyst or the SCRF™ catalyst (i.e. as a separate substrate monolith),or more preferably a zone on a downstream or trailing end of thesubstrate monolith comprising the SCR catalyst can be used as a supportfor the ASC.

Generally, it is preferred that the exhaust system of the invention doesnot comprise a lean NO_(x) trap (LNT) (sometimes referred to as a leanNO_(x) trap catalyst, a NO_(x) adsorber catalyst (NAC), a De NO_(x) trap(DNT) catalyst, a NO_(x) storage catalyst, or a NO_(x) storage/reduction(NSR) catalyst).

The invention further provides a vehicle comprising a compressionignition engine and either an exhaust system of the invention or anoxidation catalyst of the invention. Generally, the compression ignitionengine is a diesel engine. The diesel engine may be a homogeneous chargecompression ignition (HCCI) engine, a pre-mixed charge compressionignition (PCCI) engine or a low temperature combustion (LTC) engine. Itis preferred that the diesel engine is a conventional (i.e. traditional)diesel engine.

The vehicle may be a light-duty diesel vehicle (LDV), such as defined inUS or European legislation. A light-duty diesel vehicle typically has aweight of <2840 kg, more preferably a weight of <2610 kg.

In the US, a light-duty diesel vehicle (LDV) refers to a diesel vehiclehaving a gross weight of 8,500 pounds (US lbs). In Europe, the termlight-duty diesel vehicle (LDV) refers to (i) passenger vehiclescomprising no more than eight seats in addition to the driver's seat andhaving a maximum mass not exceeding 5 tonnes, and (ii) vehicles for thecarriage of goods having a maximum mass not exceeding 12 tonnes.

Alternatively, the vehicle may be a heavy-duty diesel vehicle (HDV),such as a diesel vehicle having a gross weight of >8,500 pounds (USlbs), as defined in US legislation.

The invention also relates to several methods and uses involving theoxidation catalyst. A general aim of all of the methods or uses of theinvention is treat an exhaust gas from a compression ignition enginewithout producing a substantial amount of nitrous oxide (N₂O), whetherthe exhaust gas is treated by (i) treating (e.g. oxidising) carbonmonoxide (CO) and hydrocarbons (HCs) in the exhaust gas, (ii) modulatingthe content of NO_(x) in the exhaust gas, and/or (iii) using theoxidation catalyst as a passive NO_(x) absorber (PNA).

The term “substantial amount of nitrous oxide (N₂O)” in this context, atleast for light duty vehicles, refers to >0.030 g/mile of N₂O asmeasured using the Federal Test Procedure 75 (FTP-75), preferably >0.025g/mile, more preferably >0.020 g/mile, such as >0.015 g/mile, and evenmore preferably 0.010 g/mile (e.g. >0.005 g/mile. Any reference to“without producing a substantial amount of nitrous oxide” in the contextof a treated exhaust gas or to “a treated exhaust gas that does notcontain a substantial amount of nitrous oxide” may refer to the exhaustgas that is passed into the atmosphere (i.e. as the exhaust gas that haspassed through an exhaust system, such as measured at the outlet of avehicle exhaust pipe) or to the exhaust gas directly obtained from theoutlet of the oxidation catalyst (i.e. when there are downstreamemissions control devices that may generate N₂O).

In general, the method aspects of the invention may include a step ofproducing a treated exhaust gas that does not contain a substantialamount of nitrous oxide (N₂O). This step typically follows the step ofcontacting the exhaust gas with the oxidation catalyst. This steptypically also precedes any step of passing the treated exhaust gas toan emissions control device. Similarly, the oxidation catalyst of theinvention can be used to produce a treated exhaust gas that does notcontain a substantial amount of nitrous oxide (N₂O).

In the methods and uses of the invention, typically the composition ofthe exhaust gas contacted with the oxidation catalyst is not altered(e.g. by changing or cycling the mode of operation of the compressionignition engine to substantially alter the air to fuel ratio (i.e. fromlean to rich or vice-versa)) to facilitate storage of NO_(x) from theexhaust gas or to release NO_(x) from the oxidation catalyst.

The methods or uses of the invention may also include a step of removingsulphur stored in the oxidation catalyst. Typically, sulphur can beremoved under lean conditions at an appreciable rate for practical usein automotive aftertreatment systems when the oxidation catalyst has atemperature greater than 550° C. The oxidation catalyst may reach thistemperature via transfer of heat from the exhaust gas. It may, however,be necessary to heat the oxidation catalyst to a temperature greaterthan 700° C., for example to 780° C. or more, to remove stored sulphur.

Generally, the oxidation catalyst may be used in combination with anemissions control device. Typically, the oxidation catalyst is used incombination with:

-   (i) a selective catalytic reduction (SCR) catalyst, preferably a SCR    catalyst downstream of the oxidation catalyst, particularly when the    oxidation catalyst is or is used as a diesel oxidation catalyst    (DOC);-   (ii) a selective catalytic reduction filter (SCRF™) catalyst,    preferably a SCRF™ catalyst downstream of the oxidation catalyst,    particularly when the oxidation catalyst is or is used as a diesel    oxidation catalyst (DOC);-   (iii) a diesel particulate filter (DPF) or a catalysed soot filter    (CSF), preferably a DPF or CSF downstream of the oxidation catalyst,    particularly when the oxidation catalyst is or is used as a diesel    oxidation catalyst (DOC);-   (iv) a diesel oxidation catalyst (DOC), preferably a DOC upstream of    the oxidation catalyst of the invention, particularly when the    oxidation catalyst is or is used as a catalysed soot filter (CSF);-   (v) a diesel particulate filter (DPF) or a catalysed soot filter    (CSF) and a selective catalytic reduction (SCR) catalyst, preferably    a DPF or CSF downstream of the oxidation catalyst and an SCR    catalyst downstream of the DPF or CSF, particularly when the    oxidation catalyst is or is used as a diesel oxidation catalyst    (DOC);-   (vi) a diesel oxidation catalyst (DOC) and a selective catalytic    reduction (SCR) catalyst, preferably a DOC upstream of the oxidation    catalyst of the invention and a SCR catalyst downstream of the    oxidation catalyst of the invention, particularly when the oxidation    catalyst is or is used as a catalysed soot filter (CSF); or-   (vii) a selective catalytic reduction (SCR) catalyst and either a    diesel particulate filter (DPF) or a catalysed soot filter (CSF),    preferably a SCR catalyst downstream of the oxidation catalyst and a    DPF or CSF downstream of the SCR catalyst, particularly when the    oxidation catalyst is or is used as a diesel oxidation catalyst    (DOC).

When the oxidation catalyst is used as a passive NO_(x) absorber (PNA),the oxidation catalyst absorbs or stores NO_(x) from the exhaust gas ata first temperature range and releases NO_(x) at a second temperaturerange, wherein the second temperature range is higher the firsttemperature range (e.g. the midpoint of the second temperature range ishigher than the midpoint of the first temperature range). It ispreferable that the second temperature range does not overlap with thefirst temperature range. There may be a gap between the upper limit offirst temperature range and the lower limit of the second temperaturerange.

Typically, the oxidation catalyst releases NO_(x) at a temperaturegreater than 200° C. This is the lower limit of the second temperaturerange. Preferably, the oxidation catalyst releases NO_(x) at atemperature of 220° C. or above, such as 230° C. or above, 240° C. orabove, 250° C. or above, or 260° C. or above.

The oxidation catalyst absorbs or stores NO_(x) at a temperature of 200°C. or less. This is the upper limit of the first temperature range.Preferably, the oxidation catalyst absorbs or stores NO_(x) at atemperature of 195° C. or less, such as 190° C. or less, 185° C. orless, 180° C. or less, or 175° C. or less.

The oxidation catalyst may preferentially absorb or store nitric oxide(NO). Thus, any reference to absorbing, storing or releasing NO_(x) inthis context may refer absorbing, storing or releasing nitric oxide(NO). Preferential absorption or storage of NO will decrease the ratioof NO:NO₂ in the exhaust gas.

In addition or as an alternative to using the oxidation catalyst as apassive NO_(x) absorber (PNA), the oxidation catalyst can be used tomodulate the content of NO_(x) in an exhaust gas from a compressionignition engine, such as for a downstream emissions control device.

Any reference to “modulate the NO_(x) content” as used herein,particularly in relation to method or use aspects of the invention,refers to changing (i.e. adjusting) or maintaining the ratio (in ppm or% volume, typically at the temperature and pressure of the exhaust gas)of NO:NO₂ to be within a predefined range at a specific exhaust gastemperature or temperature range.

In general, “modulate the NO_(x) content” refers to changing ormaintaining, preferably changing, the ratio (in ppm or % volume) ofNO:NO₂ in an exhaust gas, typically directly from the compressionignition engine, to be less than 17:3 (i.e. the amount of NO to NO₂ isless than that which is normally found in an exhaust gas from acompression ignition engine), preferably the ratio of NO:NO₂ is from 5:1to 1:5, more preferably 2.5:1 to 1:2.5, and even more preferably 2:1 to1:2 (e.g. 1.5:1 to 1:1.5 or about 1:1). The ratio of NO:NO₂ when thetemperature is at the first temperature range (i.e. the temperature atwhich NO_(x) is stored or absorbed) may be lower than the ratio at thesecond temperature range (i.e. the temperature at which NO_(x) isreleased).

In the second method aspect of the invention, the step of “(a)controlling the NO_(x) content of an exhaust gas by contacting theexhaust gas with an oxidation catalyst . . . ” may further include thesteps of (i) absorbing or storing NO_(x) from the exhaust gas at a firsttemperature range, and (ii) releasing NO_(x) at a second temperaturerange, thereby producing a treated exhaust gas. Preferably, the secondtemperature range is higher the first temperature range (e.g. themidpoint of the second temperature range is higher than the midpoint ofthe first temperature range). More preferably the second temperaturerange does not overlap with the first temperature range. There may be agap between the upper limit of first temperature range and the lowerlimit of the second temperature range.

Typically, the second temperature range is a temperature greater than200° C., preferably, a temperature of 220° C. or above, such as 230° C.or above, 240° C. or above, 250° C. or above, or 260° C. or above.

The first temperature range is typically a temperature of 200° C. orless, preferably a temperature of 195° C. or less, such as 190° C. orless, 185° C. or less, 180° C. or less, or 175° C. or less.

Generally, the step of (b) passing the treated exhaust gas to anemissions control device typically involves directly passing the treatedexhaust gas to the emissions control device. Thus, an outlet of theoxidation catalyst is directly connected (e.g. without intermediary) toan inlet of the emissions control device.

The emissions control device is typically a selective catalyticreduction (SCR) catalyst, a selective catalytic reduction filter (SCRF™)catalyst, a diesel particulate filter (DPF), or a catalysed soot filter(CSF).

In the second method aspect of the invention, the references to “NO_(x)content”, “absorbing or storing NO” or “releasing NO_(x)” may refer tonitric oxide (NO), such as when the oxidation catalyst preferentiallystores NO.

In the fourth use aspect of the invention, the oxidation catalyst isused in the regeneration of an emissions control device having afiltering substrate. Typically, the emissions control device having afiltering substrate is downstream of the oxidation catalyst.

The emissions control device having a filtering substrate may beselected from the group consisting of a diesel particulate filter (DPF),a catalysed soot filter (CSF), a selective catalytic reduction filter(SCRF™) catalyst and a combination of two or more thereof.

The oxidation catalyst may be used to regenerate the emissions controldevice having a filtering substrate by oxidising nitric oxide (NO) tonitrogen dioxide (NO₂) at a temperature of at least 220° C., preferablyat least 240° C., more preferably at least 260° C., still morepreferably at least 280° C.

Definitions

The term “washcoat” is well known in the art and refers to an adherentcoating that is applied to a substrate usually during production of acatalyst. The coating or washcoat generally comprises one or morecomponents of a catalyst formulation, which components are typicallyselected from a platinum group metal, a support material, an oxygenstorage component and a hydrocarbon adsorbent.

The term “washcoat region” as used herein refers to an area of washcoaton a substrate. A “washcoat region” can, for example, be disposed orsupported on a substrate as a “layer” or a “zone”. The area orarrangement of a washcoat on a substrate is generally controlled duringthe process of applying the washcoat to the substrate. The “washcoatregion” typically has distinct boundaries or edges (i.e. it is possibleto distinguish one washcoat region from another washcoat region usingconventional analytical techniques).

It is preferable that each “washcoat region” has a substantially uniformcomposition (i.e. there is no substantial difference in the compositionof the washcoat when comparing one part of the washcoat region withanother part of that washcoat region). Substantially uniform compositionin this context refers to a material (e.g. washcoat region) where thedifference in composition when comparing one part of the washcoat regionwith another part of the washcoat region is 5% or less, usually 2.5% orless, and most commonly 1% or less.

The term “washcoat zone” as used herein refers to a washcoat region ofsubstantially uniform length. The length of a washcoat zone may be thesame as the total length of the substrate. In general, the length of awashcoat zone is less than the total length of the substrate. The totallength of a substrate is the distance between its inlet end and itsoutlet end (e.g. the opposing ends of the substrate). A “washcoat zone”typically has a length (i.e. a substantially uniform length) of at least5% of the total length of the substrate.

Any reference to a “substantially uniform” in the context of a length orto “substantially uniform length” as used herein refers to a length thatdoes not deviate by more than 10%, preferably does not deviate by morethan 5%, more preferably does not deviate by more than 1%, from its meanvalue.

Any reference to a “washcoat zone disposed at an inlet end of thesubstrate” used herein refers to a washcoat zone disposed or supportedon a substrate that is nearer to an inlet end of the substrate than itis to an outlet end of the substrate. Thus, the midpoint of the washcoatzone (i.e. at half its length) is nearer to the inlet end of thesubstrate than the midpoint is to the outlet end of the substrate.Similarly, any reference to a “washcoat zone disposed at an outlet endof the substrate” used herein refers to a washcoat zone disposed orsupported on a substrate that is nearer to an outlet end of thesubstrate than it is to an inlet end of the substrate. Thus, themidpoint washcoat zone (i.e. at half its length) is nearer to the outletend of the substrate than the midpoint is to the inlet end of thesubstrate.

When the substrate is a wall-flow filter, then generally any referenceto a “washcoat zone disposed at an inlet end of the substrate” refers toa washcoat zone disposed or supported on the substrate that is (a)nearer to an inlet end of an inlet channel of the substrate than it isto a closed end of the inlet channel, and/or (b) nearer to a closed endof an outlet channel of the substrate than it is to an outlet end of theoutlet channel. Thus, the midpoint of the washcoat zone (i.e. at halfits length) is (a) nearer to an inlet end of an inlet channel of thesubstrate than the midpoint is to the closed end of the inlet channel,and/or (b) nearer to a closed end of an outlet channel of the substratethan the midpoint is to an outlet end of the outlet channel. Similarly,any reference to a “washcoat zone disposed at an outlet end of thesubstrate” when the substrate is a wall-flow filter refers to a washcoatzone disposed or supported on the substrate that is (a) nearer to anoutlet end of an outlet channel of the substrate than it is to a closedend of the outlet channel, and/or (b) nearer to a closed end of an inletchannel of the substrate than it is to an inlet end of the inletchannel. Thus, the midpoint of the washcoat zone (i.e. at half itslength) is (a) nearer to an outlet end of an outlet channel of thesubstrate than the midpoint is to the closed end of the outlet channel,and/or (b) nearer to a closed end of an inlet channel of the substratethan the midpoint is to an inlet end of the inlet channel.

Any reference to “absorbing NO_(x)” from an exhaust gas as used hereinrefers to the removal of NO_(x) from the exhaust gas by storing it inthe oxidation catalyst. The storage may be a process of adsorption, butthe oxidation catalyst is not limited to storing NO_(x) in this specificway.

The term “mixed oxide” as used herein generally refers to a mixture ofoxides in a single phase, as is conventionally known in the art.

The term “composite oxide” as used herein generally refers to acomposition of oxides having more than one phase, as is conventionallyknown in the art.

Any reference to a temperature or temperature range, such as the “firsttemperature range” or the “second temperature range” as used hereingenerally refers to the temperature of the exhaust gas, preferably thetemperature of the exhaust gas at an inlet of the oxidation catalyst.

Any reference herein to an amount in units of g ft⁻³ (grams per cubicfoot) or g in⁻³ (grams per cubic inch) refer to the mean weight of acomponent per volume of the substrate and typically includes the volumeof the void spaces of the substrate.

The expression “consisting essentially” as used herein limits the scopeof a feature to include the specified materials or steps, and any othermaterials or steps that do not materially affect the basiccharacteristics of that feature, such as for example minor impurities.For the avoidance of doubt, the expression “consisting essentially of”embraces the expression “consisting of”.

In the context of platinum (Pt) or palladium (Pd), it is to beappreciated that it is often difficult to characterise the exactcatalytic species in a catalyst and the platinum or palladium may not bepresent in elemental, metallic form. Any reference to “consistingessentially of platinum . . . ” embraces a “platinum component” wherethe platinum moiety can be an elemental form of platinum, an alloycontaining platinum or a compound comprising platinum (e.g. an oxide ofplatinum), preferably an elemental form of platinum or an alloycontaining platinum, more preferably an elemental form of platinum.Similarly, any reference to “consisting essentially of palladium . . . ”embraces a “palladium component” where the palladium moiety can be anelemental form of palladium, an alloy containing palladium or a compoundcomprising palladium (e.g. an oxide of palladium), preferably anelemental form of palladium or an alloy containing palladium, morepreferably an elemental form of palladium.

The term “substantially free” as used herein in the context of aparticular chemical entity refers to a composition or material thatcontains less than 0.5% by weight of the chemical entity, typically lessthan 0.1% by weight of the chemical entity, such as less than 0.01% byweight of the chemical entity. Generally, the chemical entity is notdetectable using conventional analytical techniques.

EXAMPLES

The invention will now be illustrated by the following non-limitingexamples.

Example 1

Palladium nitrate was added to a slurry of ceria in water. The slurrycontaining palladium nitrate was stirred to homogenise then coated ontoa metallic substrate with 300 cells per square inch using conventionalcoating techniques to form a first layer. The coated part was dried andcalcined at 500° C.

A second slurry was prepared by taking alumina powder and milling to aparticle size where the d₉₀ was less than 20 micron. Soluble platinumand palladium salts were added and the resulting slurry was stirred tohomogenise. This second slurry was coated on to the part usingconventional coating techniques to form a second layer. The part wasdried and calcined at 500° C.

The resulting catalyst had a total loading of platinum and palladium(i.e. the total PGM loading in both layers) of 160 g/ft³ and the totalmass ratio (i.e. both layers) of Pt:Pd was 5:11.

Example 2

An alumina slurry was prepared using an alumina powder that had beenmilled to a particle size where the d₉₀ was less than 20 micron. Solubleplatinum and palladium salts were added and the resulting slurry wasstirred to homogenise. This slurry was then coated on to a metallicsubstrate with 300 cells per square inch using conventional coatingtechniques to form a first layer. The part was dried and calcined at500° C.

Palladium nitrate was added to a slurry of ceria in water. This secondslurry containing palladium nitrate was stirred to homogenise and wasthen coated onto the part using conventional coating techniques to forma second layer. The coated part was dried and calcined at 500° C.

The resulting catalyst had a total loading of platinum and palladium(i.e. the total PGM loading in both layers) of 160 g/ft³ and the totalmass ratio (i.e. both layers) of Pt:Pd was 5:11.

Experimental Results

The catalysts of Examples 1 and 2 were hydrothermally aged at 800° C.for 16 hours. Each catalyst was installed on a bench mounted engine andtested for (a) its CO light off using a temperature ramp and (b) its N₂Oemissions over a simulated MVEG-B drive cycle. Emissions measurementswere continuously recorded both upstream and downstream of eachcatalyst.

The CO light off results are reported as a T₈₀, which is the temperatureat which 80% of the CO emission from the engine has been converted toCO₂. The results are shown in Table 1 below.

TABLE 1 Example No. T₈₀ (° C.) 1 120 2  96

The amount of N₂O in the exhaust emission from each catalyst afterrunning the engine over the MVEG cycle is shown in FIG. 1. The N₂Oemission when the catalyst of Example 2 was used (see the full line inFIG. 1) was lower than that from Example 1 (see the dashed line in FIG.1).

Examples 3 to 5

Palladium nitrate was added to a slurry of ceria in water. The slurrycontaining the palladium nitrate was stirred to homogenise and thencoated onto a ceramic substrate using conventional coating techniques.The coated part was dried and calcined at 500° C. The loading of ceriawas 2.7 g in⁻³ (Examples 3 to 5). The loading of palladium as apercentage of the loading of ceria was varied as follows: 1 wt %(Example 3); 2 wt % (Example 4); 4 wt % (Example 5). Each of the coatedparts was hydrothermally aged at 750° C. for 15 hours.

Experimental Results

A 1×3″ core sample from each of the catalysts of Examples 3, 4 and 5 wastaken and tested on a synthetic gas rig. The gas mix that was used toperform the tests is shown in Table 2 below.

TABLE 2 Gas Amount NO 100 ppm CO 500 ppm CO₂ 4.5% H₂O   5% O₂  12% N₂Balance

Each of the core samples was brought up to 80° C. and then exposed tothe gas mix in Table 2 for 10 minutes with the aim of storing NO_(x).The gas mix was then switched to nitrogen and each core sample washeated to 600° C. with the aim of thermally releasing any stored NO_(x).

The above procedure was repeated except that each core sample wasbrought up to 120° C., 160° C. and 200° C. before being exposed to thegas mix in Table 2. The amount of NO_(x) stored over each 10 minuteperiod was measured and the results are shown in FIG. 2.

FIG. 2 shows that increasing the loading of Pd from 1 wt % (▪) to 2 wt %(▴) increases the amount of NO_(x) that is stored. A further increase inthe Pd loading from 2 wt % (▴) to 4 wt % (●) provided a relatively smallincrease in NO_(x) storage.

Examples 6 to 8

Palladium nitrate was added to a slurry of ceria in water. The slurrycontaining palladium nitrate was stirred to homogenise and then coatedonto a ceramic substrate using conventional coating techniques. Thecoated part was dried and calcined at 500° C. The loading of palladiumwas 46.7 g ft⁻³ (Examples 6 to 8). The loading of ceria was varied asfollows: 1.35 g in⁻³ (Example 6); 0.9 g in⁻³ (Example 7); 0.675 g in⁻³(Example 8). The loading of palladium as a percentage of the loading ofceria is as follows: 2 wt % (Example 6); 3 wt % (Example 7); 4 wt %(Example 8). Each of the coated parts was hydrothermally aged at 750° C.for 15 hours.

Experimental Results

A 1×3″ core sample from each of the catalysts of Examples 3, 6, 7 and 8was taken and tested on a synthetic gas rig. The method and gas mix usedto test Examples 3 and 6 to 8 was the same as that used for Examples 3to 5 above. The results are shown in FIG. 3.

FIG. 4 shows that for a fixed Pd loading of 46.7 g ft⁻³ the amount ofstored NO_(x) storage increases with the loading of ceria. However, thistrend is not linear and smaller increases in the amount of stored NO_(x)storage were observed when the loading of ceria was increased from 1.35g in⁻³ (●) to 2.7 g in⁻³ (▴).

For the avoidance of any doubt, the entire content of any and alldocuments cited herein is incorporated by reference into the presentapplication.

1. An oxidation catalyst for treating an exhaust gas from a compressionignition engine, which oxidation catalyst comprises: a substrate; afirst washcoat region comprising palladium (Pd) and a first supportmaterial comprising cerium oxide; and a second washcoat regioncomprising platinum (Pt) and a second support material.
 2. An oxidationcatalyst according to claim 1, wherein the second washcoat regionfurther comprises palladium (Pd).
 3. An oxidation catalyst according toclaim 2, wherein the mass of platinum (Pt) is greater than the mass ofpalladium (Pd) in the second washcoat region.
 4. An oxidation catalystaccording to any one of the preceding claims, wherein the second supportmaterial comprises a refractory metal oxide.
 5. An oxidation catalystaccording to claim 4, wherein the refractory metal oxide is selectedfrom the group consisting of alumina, silica, titania, zirconia, ceriaand mixed or composite oxides of two or more thereof.
 6. An oxidationcatalyst according to claim 5, wherein the refractory metal oxide isalumina.
 7. An oxidation catalyst according to any one of the precedingclaims, wherein the first washcoat region comprises an amount ofpalladium (Pd) of less than 2% by weight, preferably 0.25 to 1.9% byweight.
 8. An oxidation catalyst according to any one of the precedingclaims, wherein the first washcoat region is a first washcoat zone andthe second washcoat region is a second washcoat zone, and wherein thesecond washcoat zone is disposed or supported on the substratedownstream of the first washcoat zone.
 9. An oxidation catalystaccording to any one of claims 1 to 7, wherein the first washcoat regionis a first washcoat layer and the second washcoat region is a secondwashcoat layer, and wherein the first washcoat layer is disposed orsupported on the second washcoat layer.
 10. An oxidation catalystaccording to any one of the preceding claims, wherein the secondwashcoat region comprises an amount of platinum (Pt) of 0.2 to 15% byweight.
 11. An exhaust system comprising an oxidation catalyst accordingto any one of claims 1 to 10, and an emissions control device.
 12. Avehicle comprising a compression ignition engine and either: (a) anoxidation catalyst according to any one of claims 1 to 10; or (b) anexhaust system according to claim
 11. 13. Use of an oxidation catalystaccording to any one of claims 1 to 10 to treat an exhaust gas from acompression ignition engine without producing a substantial amount ofnitrous oxide (N₂O).
 14. Use of an oxidation catalyst according to anyone of claims 1 to 10 as a passive NO_(x) absorber (PNA) in an exhaustgas from a compression ignition engine optionally in combination with anemissions control device.
 15. A method of modulating the content ofNO_(x) in an exhaust gas from a compression ignition engine for anemissions control device, which method comprises: (a) controlling theNO_(x) content of an exhaust gas by contacting the exhaust gas with anoxidation catalyst according to any one of claims 1 to 10 to produce atreated exhaust gas; and (b) passing the treated exhaust gas to anemissions control device.